SOIL ORGANISMS 407 



is yet to be determined, and will depend on a number of fac- 

 tors. The soil must, of course, be deficient in the constituent 

 composted with sulfur. Otherwise, an application of sulfur 

 alone would give just as good results. Again the cost of 

 composting must be reckoned with. It yet remains to be 

 proven by crop growth whether the efficiency of sulfur is any 

 greater when it is composted with such materials as raw rock 

 phosphate and farm manure and applied to the soil, than 

 when these materials are added separately. 



225. The mineral cycle. — The strictly mineral constitu- 

 ents of the soil seem to undergo as complex and intricate 

 transformations as do the elements that are considered as 

 more closely related to the soil organic matter, such as car- 

 bon, nitrogen and sulfur. While a part of the mineral cycle 

 is purely chemical or physico-chemical, the biological phase 

 is by no means unimportant. In fact, were it not for the in- 

 fluence of organisms within the soil, little or no mineral mat- 

 ter, such as phosphoric acid and potash, would ever become 

 available to higher plants. 



When plant or animal tissue enters the soil, it undergoes 

 decay in the manner already described, the ash constituents 

 being liberated and either utilized directly by higher plants 

 again or converted into a part of the soil mass. The main 

 source of the mineral nutrients for any plant is of course the 

 inorganic portion of the soil rather than the organic part. 

 It is thus necessary to investigate what influence, if any, soil 

 organisms have on such material. 



The action of organisms on the inorganic portions of the 

 soil is of two kinds: (1) direct, and (2) indirect. In the 

 former the soil organisms themselves attack the mineral mat- 

 ter, rendering part of it available. Some of this soluble ma- 

 terial is absorbed by the organisms, becoming a part of the 

 cell contents. When the fungus or bacterium dies, this ma- 

 terial through decay again becomes available and may be 

 used by higher plants. While most soil organisms probably 



